1. Field of the Invention
The present invention relates to a backlight unit and a liquid crystal display device employing the same and, more particularly, to a direct light type backlight unit having a structure in which divided luminance areas are sequentially turned on and off and a liquid crystal display device employing the backlight unit.
2. Description of the Related Art
A liquid crystal display (LCD), which is one type of flat panel display, is a passive display that forms an image without using self-luminescence but instead using incident light from the outside. A backlight unit is disposed at a rear of the LCD to irradiate light toward the liquid crystal panel.
A cold cathode fluorescence lamp (CCFL) is generally used as a light source for backlight units. However, the CCFL has a comparatively short lifespan and inferior color reproducibility. It is also impossible instantly to turn on the CCFL. Therefore, when a backlight unit using the CCFL is divided into a plurality of luminance areas, the luminance areas cannot be sequentially turned on in synchronization with a screen scanning of a liquid crystal device. Hence, it is difficult effectively to prevent a motion blur in which after-images remain when an image frame is transferred to another image frame in a liquid crystal display device.
To overcome such drawbacks of the CCFL, a backlight unit using a point light source, such as a light emitting diode (LED), recently has been developed instead of the CCFL. Examples of such a backlight unit are disclosed in Korea Patent Application No. 10-2003-0019834, entitled “STRUCTURE OF BACKLIGHT UNIT FOR LIQUID CRYSTAL DISPLAY”, and Korea Patent Application No. 10-2003-0023052, entitled “STRUCTURE OF BACKLIGHT UNIT FOR LIQUID CRYSTAL DISPLAY”.
In Korea Patent Application No. 10-2003-0019834, a light guide panel is divided into a plurality of divided blocks, and a light emitting diode is mounted on each side of the divided blocks. Groove-shaped boundary regions corresponding to the boundaries between divided blocks are formed on a rear side of the light guide plate, and thus leaking of light emitted from the LED into adjacent blocks is prevented. Meanwhile, in Korea Patent Application No. 10-2003-0023052, a light guide plate is also divided into a plurality of blocks, and an LED is mounted at a side surface of each of the divided blocks. However, instead of forming a groove at a boundary between divided blocks of the light guide panel, each of the divided blocks is formed to have a refractive index decreasing from a central part to a boundary part for preventing the light emitted from the LED from leaking into adjacent blocks.
However, the above mentioned conventional backlight units are edge light type backlight units in which a light source is mounted at an edge of a light guide panel (LGP) and light emitted from the light source is transferred to a liquid crystal panel. The edge light type backlight unit has a difficulty to transfer uniformly the light to the liquid crystal panel and has a degraded light efficiency. When the LGP is divided into a plurality of blocks and the blocks sequentially emit light, it is important to prevent light emitted from a block from leaking into adjacent blocks. However, the conventional backlight units are not able perfectly to prevent light leakage. Particularly, when the refractive index of each block of the light guide panel gradually changes over the block, there must be large light loss in the block and a manufacturing cost of the light guide panel increases. Also, a comparatively large amount of light leaks into the adjacent areas.